1. Field of the Invention
The present invention relates generally to audio amplifiers, and more specifically, to techniques for reducing offset generated between a digital audio input and the amplifier output.
2. Background of the Invention
Audio amplifiers typically exhibit offset due to internal offsets in the amplifier and/or due to offset present in the input signal that is passed to the output of the amplifier. In systems having a digital input, the no-signal level can generally be relied upon to have no offset for most program content, as the processing of the program content generally includes removing any DC offset. However, once the digital signal has been converted to an analog output, the problem of circuit-generated offset remains.
In particular, in circuits using switched resistor networks to control the gain of one of the audio amplifiers in the signal chain, there is a trade-off between resistor size and allowable offset, for a given gain selection resolution and accuracy. In such circuits, it is generally necessary to allow some offset to be present, or the resistor area required will become too large.
DC offset at an analog audio output also wastes power, reduces dynamic range and presents a problem of generating pops when the analog audio output is enabled, disabled or when the gain of an audio amplifier that amplifies the offset is changed. In present-day battery operated devices using high-efficiency headphone drivers, even a small offset, e.g. 10 mV, can waste power at a level comparable to that required to operate the device at nominal listening levels.
Therefore, it would be desirable to provide techniques for removing offset from an analog audio output signal.